It's hard to argue that there's a better virtual assistant for the iPhone when you've got Siri, because Apple can create app that is completely integrated with iOS. That said, if you don't have Siri there are plenty of great options that may even be better some day. Here's a look at why Siri's great but plenty of other excellent alternatives as well.

Siri

Platform: iPhone 4SPrice: Free (with an iPhone 4S)Built-in

Tell Siri what you want with voice commands, and it will respond to you with an answer or ask you for more information.

Yesterday Apple announced Siri, the new voice-controlled assistant for your iPhone, and we detailed …
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Siri primarily excels in two ways: it's fully integrated with your iPhone so it can do almost anything and you don't need to have the app open for its complete functionality. Both of these things are advantages that Apple has over the competition because it can integrate its apps much more closely with the operating system than any third party will ever be able to do. That said, Siri is also great because it can do more than most third-party apps can do anyway. It's capable of looking up information, making complex calculations (via Wolfram Alpha), getting driving directions, making calls and texts, adding appointments to your calendar, and much more. It can even tell you a joke. Siri is so thorough that even if you combine every third party app listed in the competition section below, they still wouldn't have all the functionality Siri can provide. That said, Siri isn't perfect and some of the other apps definitely have their advantages. It pretty much comes down to this: If you have Siri, it's most likely your best choice in virtual assistants. If you don't, there are some really great alternatives.

Siri has several downsides. First, it only (currently) works on an iPhone 4S (although there's a complicated workaround for jailbroken devices). Second, it doesn't work hands-free (although there's a jailbreak workaround for that, plus a non-jailbreak trick you can use to avoid pushing the home button). Perhaps most importantly, Siri sometimes has some issues with accuracy. This should be expected with any voice recognition software, but it seems to misunderstand certain words more so than some of the competition listed below. That said, Siri's level of accuracy is still very good, but if you're having trouble getting it to understand you then you might want to see how another app fares. Finally, although Siri is incredibly well-integrated into iOS, there is still more it could do—and jailbreak hacks make these things possible. For example, launching apps, changing settings, and other custom commands aren't available by default, but you can definitely add them if you're jailbroken.

GitHub user plamoni has created a means to add custom commands to Siri, including the ability the…
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Vokul ($3) is a virtual assistant geared more towards using your phone safely while you drive. While it's great for that, there's no reason you can't use it as an assistant in other situations as well. It can send email, texts, make calls, and play your music. What it can't do, at the moment, is get you driving directions or search the web for information. That said, it has, by far, the most accurate voice recognition of any app I've tested so far. Read more about Vokul here.

Evi ($1) fills in the functionality that Vokul doesn't have. While it can't (yet) send emails or texts or do much in the way of controlling your phone, it can look up just about any information you need online—and you can use your voice or search manually. For example, if you wanted to know what time it is in Beijing or when Thanksgiving is in 2016, Evi can find out for you. You can ask it just about anything. If it can't find the answer, it'll try to provide you with relevant information. For example, it wasn't sure if it was a leap year this year so it pointed me to the leap year page on Wikipedia.

Vlingo (Free), in theory, is a perfect option. It can get directions, send emails, compose text messages, update Facebook and Twitter, search the web, and make phone calls—all using the power of your voice. In my tests, however, it had a very hard time finding accurate results. This hasn't been the case with other apps, so I have to assume the voice recognition just isn't as good. That said, Vlingo is free and a lot of people use it, so there's certainly no harm in trying to see if it works well for you. If it does, it's pretty much the best deal on virtual assistants you'll find in the iTunes App Store.

Voice Actions ($5) is an app you should probably avoid. Not only is it more expensive than your other options, but it seems to get confused by most commands. It's supposed to be capable of finding all sorts of information, setting alarms, voice dialing contants, and much more, but even when it understood the names I gave it—both simple and complex—it wasn't sure what to do with them. I'd say "call Whitson Gordon" and it would get the name right, but search for it rather than recognize I wanted to make a phone call. While Voice Actions can supposedly do quite a bit, I couldn't get it to do all that much. Your mileage may vary, but considering it's the most expensive option you've got it's hard to recommend giving it a try when you've got a free app that does the same stuff and two cheaper paid apps that work very well.

Lifehacker's App Directory is a new and growing directory of recommendations for the best applications and tools in a number of given categories.